CIS-60 Home http://www.c-jump.com/CIS60/CIS60syllabus.htm

C++ flow control

1. Structured programming and C++ flow control
2. if-else statement
3. if-else statement example
4. else-if construct
5. else-if example
6. while loop
7. for loop
8. for loop timeline
9. for loop
10. for loop example
11. forever
12. do while loop
13. switch statement
14. switch statement considerations
15. break statement
16. continue statement
17. Conclusion

Structured programming and

C++ flow control

• if statement
• else if construct
• while loop
• for loop
• switch statement
• break statement

if-else statement

        if ( condition ) { 

            statement true; 

        } else { 

            statement false; 

        } 

if-else statement example

#include <iostream>
void main()
{
    int x = -1;
    
    if ( x > 0 ) { 
    
        std::cout << "x is a positive number"; 
        
    } else { 
    
        std::cout << "x is a negative or zero"; 
    } 
}

else-if construct

(else-if ladder)

    if ( condition-1 ) {

        statement; // condition-1 is true

    } else if ( condition-2 ) {

        statement; // condition-2 is true

    } else if ( condition-3 ) {

        statement; // condition-3 is true

    } else {

        // default case:
        statement; // all above conditions were false

    }

else-if ladder example

#include <iostream>
void main()
{
    int x = -1;
    
    if ( x > 0 ) { 
    
        std::cout << "x is positive"; 
        
    } else if ( x < 0 ) { 
    
        std::cout << "x is negative"; 

    } else { 
    
        std::cout << "x is zero"; 
    } 
}

while loop

    while ( expression ) {
        statement;
    }

• The expression is evaluated;
• If it is non-zero, statement is executed;
• The expression is re-evaluated.
• Therefore, while continues cycling until expression becomes zero:

        void main()
        {
            int x = 0;
            while ( x < 5 ) { // condition
                // loop body:
                x = x + 1;
            }  
        }

for loop

• The for statement

  
      for ( expr1; expr2; expr3 ) {
          statement;
      }
      
  

  is similar to

     expr1;
     while ( expr2 ) {
         statement;
         expr3;
     }

for loop timeline

        for ( init; condition; increment ) {
            statement;
        }
        ...
    

• The timeline of execution goes like this:

      for ( init; true; increment ) { statement; } ...
      for ( init; true; increment ) { statement; } ...
      for ( init; true; increment ) { statement; } ...
      for ( init; true; increment ) { statement; } ...
      for ( init; true; increment ) { statement; } ...
      for ( init; true; increment ) { statement; } ...
      for ( init; true; increment ) { statement; } ...
      for ( init; false;increment ) { statement; } ...
      for ( init; false;increment ) { statement; } ...
  

for loop

#include <iostream>
void main()
{
    int x = 0;
    for ( x = 0; x < 5; ++x ) {
        std::cout << x;
    }
}

for loop example

#include <iostream>
void main()
{
    int x = 0;
    for ( x = 0; x < 5; ++x ) {
        std::cout << x;
    }
}

forever

#include <iostream>
void main()
{
    for ( ;; ) // forever
    {
        // "endless" loop:
        char answer;
        std::cout << "Exit program? Y/N:";
        std::cin >> answer;
        if ( answer == 'Y' || answer == 'y' ) {
            return;
        }
    }
}

do...while loop

• The while and for loops test the termination condition at the top.
• By contrast, the third loop in C++, the do-while,
  tests condition at the bottom:

  
          #include <iostream>
          void main()
          {
              int idx = 0;
              // Prints 01234:
              do {
                  std::cout << idx;
              } while ( ++idx < 5 );
          }
  
  

switch statement

#include <iostream>
int main()
{
    std::cout << "Do you want to proceed (y or n)?\n";
    char answer = 0;
    std::cin >> answer; // get answer from user

    switch ( answer ) {
    case 'y':
        return true;

    case 'n':
        return false;

    default:
        std::cout << "Sorry, I don't understand\n";
        std::cout << "I'll take that for a no";
        return false;
    }
}

• case constants must be distinct
• default is optional

switch statement considerations

#include <iostream>
void main()
{
    bool proceed = false ;
    std::cout << "Do you want to proceed (y or n)? "; // display question
    char answer = 0;
    std::cin >> answer; // read answer
    switch ( answer ) {
        case 'Y':
        case 'y':
            proceed = true;
            break;
        default:
            std::cout << "Sorry, I don't understand\n";
            std::cout << "I'll take that for a no\n";
        case 'N':
        case 'n':
            proceed = false;
            break;
    }
}

• cases and the default can occur in any order
• break statement causes an immediate exit from the switch
• common ways to leave a switch - break and return
• put a break after each case, including default
• fall-throughs should be used sparingly, and well commented!

break statement

• break causes an immediate exit from the switch
• break also causes exit from while and for:

          void main()
          {
              int x = 0;
              while ( x < 5 ) { // condition
                  // loop body:
                  x = x + 1;
                  if ( x == 3 )
                      break;
              }  
          }

• break cannot be used to exit if-else code blocks:

          void main()
          {
              int x = 0;
              if ( x < 5 ) {
                  break; // Error: illegal break
              }  
          }

continue statement

• The continue statement causes next loop iteration to begin:
  • Inside while loop this means that the test part is executed immediately;

  • Inside for, control passes to the increment step.

• The continue statement applies only to loops, not to switch.
• For example,

          for ( i = 0; i < n; ++i ) {
              int result = get_data( i );
              if ( result < 0 ) // skip negative values
                  continue;
              // process zero and positive values...
          }

Conclusion